Add tests for webhook and fix some webhook bugs (#33396)

This PR created a mock webhook server in the tests and added integration
tests for generic webhooks.
It also fixes bugs in package webhooks and pull request comment
webhooks.

services/webhook/matrix.go

@@ -24,6 +24,10 @@ import (
 	webhook_module "code.gitea.io/gitea/modules/webhook"
 )
 
+func init() {
+	RegisterWebhookRequester(webhook_module.MATRIX, newMatrixRequest)
+}
+
 func newMatrixRequest(_ context.Context, w *webhook_model.Webhook, t *webhook_model.HookTask) (*http.Request, []byte, error) {
 	meta := &MatrixMeta{}
 	if err := json.Unmarshal([]byte(w.Meta), meta); err != nil {